1. Field of the Invention
The present invention relates to a drive circuit of an oscillation gyro which includes two piezoelectric elements formed in an oscillator.
2. Description of Related Art
As a civilian gyro used for detection of an unintentional movement of the hands in taking an image when using a video camera, direction detection in a car navigation system, attitude control of a movable body, such as a vehicle, and the like, a columnar oscillation gyro utilizing a flexing oscillation has been used. For example, an oscillation gyro in which two piezoelectric elements are adhered to a quadrangular prism-like oscillator is put to practical use (for example, Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2000-304543).
Heretofore, for example, as shown in FIG. 1, a drive circuit 20 drives an oscillation gyro 1 which includes a quadrangular prism-like oscillator 2, a piezoelectric element 3a and a piezoelectric element 3b, and is formed in such a manner that the piezoelectric element 3a and the piezoelectric element 3b are aligned on a first side face of the oscillator 2. The surface of the oscillator 2 is conductively plated. Then, a detection signal is outputted from outputs of the two piezoelectric elements 3a, 3b. 
In the drive circuit 20 of the conventional oscillation gyro 1 shown in FIG. 1, a second side face opposed to a first side face of the oscillator 2 is connected to a reference potential Vref. This reference potential Vref is applied to a side face that is brought into contact with the oscillator 2 of the piezoelectric element 3a and the piezoelectric element 3b through the oscillator 2.
To the piezoelectric element 3a and the piezoelectric element 3b provided on the first side face of the oscillator 2, an adding circuit 22 and a differential amplifier 26 are respectively connected, and an oscillation circuit 24 is connected through resistors 21a, 21b and a capacitor 25. Also, the adding circuit 22 is connected to a comparing circuit 23, which is connected to the oscillation circuit 24.
The differential amplifier 26 is connected to a synchronous detector 27, and a detection signal Vsd′ by this synchronous detector 27 is outputted as a detection signal through a DC amplifier 28.
In the drive circuit 20 of such a configuration, an output signal Vgo′ of the oscillation circuit 24 is supplied to the side faces of the piezoelectric elements 3a, 3b, opposed to the other side faces of the piezoelectric elements 3a, 3b that abut on the oscillator 2, through the capacitor 25 and the resistors 21a, 21b. 
An output signal Vgl′ of the piezoelectric element 3a and an output signal Vgr′ of the piezoelectric element 3b are added by the adding circuit 22, and its addition signal Vsa′ is inputted to the comparing circuit 23. The comparing circuit 23 compares the addition signal Vsa′ by the adding circuit 22 with the output signal Vgo′ of the oscillation circuit 24 and supplies the comparison output signal Vcm′ to the oscillation circuit 24. Also, a differential signal Vda′ between the output signal Vgl′ of the piezoelectric element 3a and the output signal Vgr′ of the piezoelectric element 3b is outputted by the differential amplifier 26 and synchronously detected by the synchronous detector 27. The synchronous detector 27 synchronously detects the differential signal Vda′ by using a clock signal Vck′ formed by the oscillation circuit 24. A detection signal Vsd′ by the synchronous detector 27 is amplified by the DC amplifier 28 and outputted as a detection signal.
FIG. 2 is a time chart of a voltage waveform in each portion of the drive circuit 20 of the conventional oscillation gyro 1 shown in FIG. 1. In FIG. 2, the case in which there is no rotation at a major axis of the oscillation gyro 1 as a center is expressed as a stationary state, and the case in which the rotation at the major axis as a center is applied is expressed as a state when a rotation angle velocity is applied.
An oscillation system is formed of a positive feedback loop including the oscillation gyro 1, the resistor 21a, the resistor 21b, the capacitor 25, the adding circuit 22, the comparing circuit 23 and the oscillation circuit 24, and it is self-oscillated in the resonance frequency of the oscillation gyro 1. The oscillation gyro 1 is flexingly oscillated in a direction perpendicular to the first side face and the second side face by this oscillation. When rotated at the major axis of the oscillation gyro 1 as a center, in this state, the direction of the flexing oscillation is changed by a Coriolis force. Thus, an output difference (Vgl′−Vgr′) is generated between the piezoelectric element 3a and the piezoelectric element 3b, and a difference signal Vda′ is obtained from the differential amplifier 26. At this time, signals for driving the oscillation gyro 1 are the outputs of the piezoelectric element 3a and the piezoelectric element 3b at a stationary state, and they are signals having the same phase and amplitude in the piezoelectric element 3a and the piezoelectric element 3b. Therefore, the signals for driving the oscillation gyro 1 are canceled by the differential amplifier 26. Then, the signals in response to the Coriolis force are the signals Vcl′ and Vcr′ of reverse phases and the same amplitude in the piezoelectric element 3a and the piezoelectric element 3b. Therefore, the difference signal Vda′ obtained by the differential amplifier 26 is a signal proportional to (Vcl′−Vcr′).
The difference signal Vda′ obtained by the differential amplifier 26 is synchronously detected thereby to be converted into the DC detection signal Vsd′. The synchronous detector 27 full-wave rectifies the difference signal Vda′ from the differential amplifier 26 at the timing of the clock signal Vck′ thereby to convert the signal Vda′ into the signal Vfr′, and then it integrates the signal Vfr′ to obtain the DC detection signal Vsd′. This detection signal Vsd′ is DC amplified to a predetermined amplitude by the DC amplifier 28, and only an angular velocity signal generated by the rotation can be detected.
The signal in response to the Coriolis force is canceled in the addition signal Vsa′ obtained by the adding circuit 22, and it becomes a signal proportional to the sum of the outputs of the piezoelectric element 3a and the piezoelectric element 3b at the stationary state. When Zgl is an impedance of the piezoelectric element 3a, Zgr is an impedance of the piezoelectric element 3b, Rb is a value of the resistor 21a and the resistor 21b, and 2·Rs is an input impedance of the adding circuit 22, the output signal Vgl′ of the piezoelectric element 3a and the output signal Vgr′ of the piezoelectric element 3b are shown as below.Vgl′=[(Zgl/Rs)/{(Zgl//Rs)+Rb}]·Vgo′={1/(1+Rb/Rs+Rb/Zgl)}·Vgo′Vgr′=[(Zgr//Rs)/{(Zrg//Rs)+Rb}]·Vgo′={1/(1+Rb/Rs+Rb/Zgr)}·Vgo′
Since the impedance Zgl of the piezoelectric element 3a and the impedance Zgr of the piezoelectric element 3b become the minimum when the oscillation gyro 1 oscillates in the resonance frequency, the voltage amplitudes of the output signal Vgl′ of the piezoelectric element 3a and the output signal Vgr′ of the piezoelectric element 3b become also the minimum. Therefore, the voltage amplitude of the addition signal Vsa′ obtained by the adding circuit 22 becomes the minimum. When the addition signal Vsa′ of the adding circuit 22 is compared with the output signal Vgo′ of the oscillation circuit 24, in a comparison output signal Vcm′ of the comparing circuit 23, the voltage amplitude becomes the maximum when the oscillation circuit 24 oscillates in the resonance frequency, becoming the positive feedback loop for forming the oscillation system. The comparison output signal Vcm′ of the comparing circuit 23 is a signal proportional to (Vgo′−Vsa′). Since the signal Vcm′ is distorted in waveform, the signal Vcm′ is waveform shaped to a triangular wave by the oscillation circuit 24 and outputted as an output signal Vgo′.
Since the DC bias voltage of the signal Vgo′ is cut in the conventional drive circuit 20, the capacitor 25 is necessary. If the capacitor 25 is not included, the DC bias voltage of the signal Vgo′ is inputted to the comparing circuit 23 through the adding circuit 22, and the DC bias voltages become the same values at the two input terminals of the comparing circuit 23. Then, the output signal Vgo′ of the oscillation circuit 24 is saturated, and hence it is not oscillated.